Orthogonal Frequency Division Multiplexing (OFDM) transmitting and receiving systems and methods are well known for voice and/or data communication. In general, OFDM is a spread spectrum technique that distributes data over a large number of carriers that may be spaced apart at various frequencies. More specifically, OFDM converts data input in series along the time axis into data in parallel, performs an Inverse Fast Fourier Transform (IFFT) with respect to the data in parallel, then converts the inverse-transformed data in parallel into data in series, and transmits the data in series to an OFDM receiver. The OFDM receiver converts the input signal into a digital signal. In this case, the digital signal is transformed by a Fast Fourier Transform (FFT) so that the digital signal is restored to an original signal.
In the IEEE 802.11a specification, a signal is transmitted and received using OFDM. IEEE 802.11a provides a standard for wireless modems. An OFDM transmitting system transmits a short preamble and a long preamble in the first half of a packet for the purpose of synchronization of a signal. An OFDM receiving system synchronizes a signal with the preamble to demodulate an OFDM signal.
FIG. 1 is a view illustrating a packet of a general OFDM signal, and more particularly shows a packet of an OFDM signal according to IEEE 802.11a.
Referring to FIG. 1, the packet of the OFDM signal has two preambles. A period of the short preamble has 16-samples and a period of the long preamble has 64-samples. Sections t1-t10 of the packet correspond to the short preamble. Sections T1 and T2 of the packet correspond to the long preamble. The length of one sample of a preamble is 50 ns, and the whole length of the preamble is 16 μs.
In IEEE 802.11a, t1-t7 sections of the packet are assigned for the purpose of signal detection, Automatic Gain Control (AGC) and diversity selection, and three sections t8, t9, and t10 of the packet are assigned for coarse carrier frequency offset and symbol synchronization offset assumption. Two sections T1 and T2 are assigned for fine carrier frequency offset and channel assumption.
GI2 is an N/2-sample guard interval, and GI is an N/4-sample guard interval. N is the number of FFT points, and a symbol equals to the FFT point plus a variable interval. For example, when the FFT point is 64, N is 64, and the symbol having the variable interval can be 80.
As described above, the OFDM receiver uses a long preamble, and both time domain and frequency domain of the FFT may use the long preamble simultaneously. Therefore, when it is a long preamble, the OFDM receiver may perform a signal buffering so that the signal is delayed.